In a CDMA cellular system, because the same carrier frequency is used repeatedly in every cell there is no need for handovers between frequencies within the same system. However, considering a case such as when existing systems are present together, there is a need for handovers between different carrier frequencies. Three points pertaining to detailed cases are described below.
As a first point, in a cell where there is considerable traffic, a separate carrier frequency is used to accommodate the increased number of subscribers, and a handover may be performed between those cells. As a second point, when an umbrella cell constitution is used, different frequencies are allocated to large and small cells, and handovers are performed between the cells. Then, as a third point, there are cases of handovers between a third generation system, such as a W (Wideband)-CDMA system, and a second generation system, such as a current mobile telephone system.
When performing handovers in cases such as those mentioned above, it is necessary to detect the power of carriers at the different frequencies. To achieve this detection, the receiver needs to only have a structure capable of detecting two frequencies. However, this increases the size of the constitution of the receiver, or makes the constitution complicated.
Furthermore, two types of handover method may be considered: a mobile assisted handover (MAHO) and a network assisted handover (NAHO). Comparing the MAHO and NAHO methods, NAHO reduces the burden of the mobile device, but to be successful, it should be necessary to synchronize the mobile device and the base station, whereby the constitution of the base station and the network becomes complicated and large in order to be capable of tracking each individual mobile device.
For such reasons, the realization of the MAHO method is more desirable, but to determine whether or not to handover, it is necessary to measure the strength of carriers of different frequencies at the mobile devices. However, a CDMA cellular system differs from a time division multiplex access (TDMA) system used in a second generation, in that it uses ordinarily continuous transmission for both transmission/reception. In this continuous transmission/reception technique, unless receivers corresponding to two frequencies are prepared, it is necessary to stop the timing of the transmission or the reception and measure the other frequency.
There has been disclosed a technique relating to a compressed mode method, for time-compressing the transmission data in the usual mode and transmitting it in a short time, thereby creating some spare time which can be utilized to measure the other frequency carrier. As an example of this, there is Japan Patent Application National Publication (Laid-Open) (JP-A) No. 8-500475 “Non-continuous Transmission for Seamless Handovers in DS-CDMA Systems”. This application discloses a method of realizing a compressed mode, wherein the spreading factor of the spreading code used is lowered to compress the transmission duration.
The method of realizing the compressed mode according to the above application will be explained below. FIG. 36 shows an example of transmissions in a normal mode and a compressed mode in a conventional CDMA system. In FIG. 36, the vertical axis represents transmission rate/transmission power, and the horizontal axis represents time. In the example of FIG. 36, the compressed mode transmission is inserted between normal transmission frames.
In the transmission in the compressed mode, a non-transmission timing is provided in the downlink frame, and can be set to a desired period of time (duration). This non-transmission timing represents idle period during which the strength of the other frequency carrier is measured. In this way, slotted transmission can be achieved by inserting the idle period during transmission of compressed mode frames.
In this type of compressed mode transmission, transmission power increases in accordance with the time ratio between the idle period and the frame (compressed mode frame) transmission timing, and therefore, as shown in FIG. 36, the compressed mode frame is transmitted at a higher transmission power than the frame in normal transmission. As a consequence, transmission quality can be maintained even in frame transmission in compressed mode.
In addition to the application mentioned above, as an example of pertinent literature there is Gustafsson, M. et al: “Compressed Mode Techniques for Inter-Frequency Measurements in a Wide-band DS-CDMA System”, Proc. of 8th IEEE PIMRC '97. This research paper discloses techniques for realizing compressed mode in cases other than when the spreading factor is lowered, namely when the coding rate is increased, when multi-code transmission is used, and when a multi-bit transmission modulation system such as 16QAM is used.
However, in conventional examples such as the application mentioned above, since transmissions are interleaved in units of one frame and within one frame, the interleaving time for slotted transmission (in the compressed mode) is more compressed than in normal transmission. Consequently, the interleaving size is shortened which leads to a problem of poor decoding at the reception side.
Furthermore, in conventional examples such as the literature mentioned above, since the length of interleaving time is shortened when using compressed mode transmission, there is increased deterioration of signal quality with respect to fading, and, since no TPC (transmission power control) command bit is sent during non-transmission, it is not possible to achieve high-speed TPC, leaving a subsequent problem of poor signal quality.
Furthermore, in conventional examples such as the application and literature mentioned above, the spreading factor is lowered when carrying out a compressed mode transmission. However, in general, lowering of the spreading factor indicates that a spreading code having a short code-length is being used. However, since the number of spreading codes that can be used is directly proportional to the square of the code-length, there is a problem that there are extremely few spreading codes having short code-lengths, and these spreading code resources, which are vital for realizing compressed mode transmission, are consumed.
It is an object of the present invention to solve the problems described above by providing a spread spectrum communication device and a spread spectrum communication method capable of preventing deterioration in signal quality caused by compressed mode, with respect to interleaving, transmission power control, spreading code allocation methods and the like to minimize the effects of transmission errors.